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2.0 .- - . ~ 1.8 ~ 1.6 0::: ~ 1.4 ~ 1.2 J: en Z 1.0 ~ ~ 0.8 ill 0.6 .....J z 0 0.4 en Z w 0.2 ~ Ei 0.0 5 1000 ----r-- T :-~Tli --i-j_-Lr-r 1 I I I i ,- r-- - l----t-ftl I i I-{---+---t-tl ! MJ~ II I: I I~I --t-l ! ! 2 3 5 10000 DISCHARGE (cfs) FIGURE 10. Variation in dimensionless grain shear stress (T:) at cross section 0.8 for two values of dimensionless critical shear stress (T.) at discharges ranging from 500 to 10,000 cis in the right branch channel, RM 16.5, Yampa River. The median size of the bed material is 68 mm (2.7 in.). the flows from both branch channels. However, in 1992 at a discharge of about 600 ds, eight male Colorado squawfish, four of which were ripe and tuberculated (E. Wick, National Park Service, unpublished data) were captured in this eddy. The hy- draulic model results (Figure 10) indicated that at this discharge the bed material should have been mobile (7'*' > 1). A similar analysis at the tertiary bar at cross section 2.7 (Figure 3), where the D50 of the bed material is 64 mm (2.5 in.), in- dicates that when a 7' *c value of 0.03 is used, the bed material is always mobile (7'*' > 1.0) at flows between 500 and 10,000 ds, but if a value of 0.047 is used, 7'*' falls be- low 1.0 at a discharge of about 4,200 ds (Figure 11). The highest dimensionless grain shear stress occurs at a discharge of about 1,200 ds when the hydraulic gra- dient is steepest (Figure 7) and decreases with increasing discharge up to about 5,000 ds where it again begins to increase. At the time of field sampling of the bed ma- terial in 1991 when the discharge in the river was about 1,200 ds, the hydraulic analysis indicated that the bed material should have been mobile and this was con- firmed by the field observations. Four adult I~ 124 Colorado squaw fish (one tuberculated fe- male and three ripe tuberculated males) were captured in the pool below the ter- tiary bar. At the tertiary bar at the head of the right branch channel (Figure 3, cross section 5.1), where the D50 of the bed material is 74 mm (2.9 in.), the bed material is mobile at dis- charges less than 2,500 ds (7'*c = 0.047) to 4,200 ds (7'*c = 0.03) (Figure 12). The hy- draulic gradient is reduced with increased discharge above about 2,000 ds (Figure 7). At the time of the field sampling of the bed material in 1991 at a discharge of about 1,200 ds, the hydraulic analysis indicated that the bed material should have been mo- bile (7'*' > 1.0) and this was confirmed by the field observations. Four adult male (ripe and tuberculated) Colorado squaw fish were captured in the pool associated with this tertiary bar. Four adult Colorado squaw fish (three ripe tuberculated males) also were captured in the pool upstream of cross sec- tion 5.1. Analysis of the variations in dimension- less grain shear stress over a range of dis- charges at the three riffles or tertiary bars in the left branch channel showed similar patterns to the right branch channel. At Rivers . Volume 4, Number 2 April 1993